“…For example, in the unphysical example (6.7) the curves span the intervals [1,4], [3,6] and [2,5]. 21 The first and the last of these intersect in [2,4] which is not one of the spanned intervals, hence this configuration is excluded as unphysical. In contrast, in the example (6.8) the curves span the set of intervals { [1,4], [2,3], [5,6]}, and apart from the empty interval the only intersection between these intervals is [2,3], which is an element of the above set.…”